The invention relates generally to the field of ceramic materials and processes for making ceramic materials. More specifically, the invention relates to silicon carbide bodies having particles or inclusions which are dispersed within the bodies.
Silicon carbide is useful in a wide variety of applications due to its tribological characteristics and its outstanding thermal, chemical and mechanical properties. Such applications include, for example, mechanical seals, valve lifters, and other applications where a part is frictionally engaged with another material. For example, in many mechanical seal applications, the seal interfaces are subjected to both a large compressive stress normal to the seal surface and to high rotational speeds or sliding velocities. Such conditions are typically represented by the parameter PV which represents the product of the compressive stress and the sliding velocity.
When such a mechanical seal is used in a pump or agitator, the mechanical seal should provide adequate sealing of the working fluid. Conveniently, the working fluid may also serve to lubricate and cool the seal interface. If sufficient lubrication and cooling is not provided, excessive wear or catastrophic failure of the mechanical seal may result. For example, if insufficient fluid is provided at the seal interface during operation, the lubricant can vaporize due to the heat produced and cause catastrophic failure.
Hence, when a silicon carbide body is used in a seal or other bearing face which runs against the face of another material, the seal or bearing face should be exposed to a lubricating and cooling fluid (or used in fluid applications) so that a film may be produced between the sliding surfaces to lubricate and cool the surfaces, thereby reducing friction, wear, and temperature as well as catastrophic failure potential. Further, proper lubrication will tend to minimize power consumption.
To facilitate proper lubrication, a variety of silicon carbide materials have been proposed. These include both reaction bonded silicon carbide materials and sintered silicon carbide materials with special modifications to the standard product. The reaction bonded silicon carbide materials are produced by placing a carbon containing preform in contact with molten silicon. As examples of such processes, U.S. Pat. Nos. 6,398,991, 4,536,449 and 4,120,731 describe reaction bonded silicon carbide bodies having secondary lubricating particles dispersed therein. The complete disclosures of these patents are herein incorporated by reference. Sintered silicon carbide materials are produced by combining a dry lubricant agglomerate and a silicon carbide matrix formulation, drying and compacting the mixture into a green body, and heating or sintering. An example of sintered silicon carbide is found in U.S. Pat. No. 5,656,563, the disclosure of which is herein incorporated by reference.
The above processes have met with limited success for a variety of reasons. For example, the processes used to produce such materials are often complex and can therefore be relatively expensive. For instance, as recognized in U.S. Pat. No. 5,656,563, it is difficult to incorporate large amounts of graphite into a ceramic matrix without causing cracks to occur in the microstructure or without increasing the material""s porosity. The use of graphite or other dry lubricants with silicon carbide bodies tends to result in unwanted lamination of the bodies. Further, crushed dry lubricants typically produce irregular-shaped inclusions in the silicon carbide body. The irregular shaped particles, and their associated sharp edges, produce stress risers within the ceramic body, resulting in a weaker ceramic body and decreased tribological performance.
Hence, it would be desirable to provide silicon carbide materials having improved strength and stability, while maintaining the lubricity of the component. It would further be desirable to provide exemplary processes for making such materials. Such processes should be relatively simple so that the overall cost of the material may be reduced. Such a silicon carbide material should also be useful in applications having a high PV value or temporary dry running applications while reducing the chances of catastrophic failure, excessive wear, and power consumption.
The invention provides exemplary silicon carbide ceramic bodies and processes for making such ceramic bodies. In one embodiment, the present invention provides a composite ceramic body having silicon carbide in major amount, and unrelated inclusions or agglomerates of a dry lubricant in minor amount. A majority of the inclusions are substantially spherical or ellipsoidal in shape, with the inclusions comprising between about two (2) percent and about twelve (12) percent of the ceramic body by volume.
In one aspect, the dry lubricant comprises graphite, and may include one or more types of graphite. Alternatively, other dry lubricants may be used, such as, boron nitride, titanium boride, aluminum nitride, and the like. The dry lubricant inclusions are dispersed throughout the silicon carbide matrix and provide a degree of lubricity when the ceramic body is operated against an operating surface.
In one aspect, the dry lubricant inclusions have an average nominal diameter or size that is between about ten (10) microns and about three hundred (300) microns. In another aspect, the inclusions comprise a primary lubricant, such as graphite, having an average size that is between about two (2) microns and about seven (7) microns. In a particular aspect, silicon carbide comprises alpha silicon carbide. Alternatively, the ceramic body includes beta silicon carbide, or some combination of alpha and beta silicon carbide. Ceramic bodies of the present invention may be used for a wide variety of applications. In a particular aspect, the ceramic body comprises a mechanical seal.
The present invention further provides exemplary raw batches for producing a composite ceramic body, and also exemplary methods for making the raw batch and ceramic body. In a particular embodiment, a raw batch of the present invention includes a silicon carbide slurry as a precursor to the silicon carbide matrix, and a plurality of agglomerates defining a dry lubricant inclusions mixture. The precursor slurry includes all needed ingredients, as known in the art, to make a dense sintered SiC ceramic body. The mixture includes a binder and a dry lubricant having an average particle size between about two (2) microns and about seven (7) microns. A majority of the agglomerates have a generally spherical shape.
In one aspect, the raw batch includes between about eighty (80) weight percent to about ninety-nine and one half (99.5) weight percent silicon carbide precursor slurry, between about one-half (0.5) weight percent to about twenty (20) weight percent dry lubricant agglomerates, and between about one-half (0.5) weight percent to about ten (10) weight percent binder. In a particular aspect, less than about three weight percent of binder is used. Again, the dry lubricant mixture includes graphite, in an embodiment, or other dry lubricant(s). In one aspect, the dry lubricant is not carbonized or carbon bonded, thus simplifying the raw batch preparation.
In alternative aspects, the binder is insoluble in water that is less than about fifty (50) degrees Celsius, or less than about ninety (90) degrees Celsius. In another aspect, the binder is soluble in water that is at least about ninety (90) degrees Celsius. In a particular embodiment, the binder has a demarcation temperature that is less than the boiling point of water, where the binder is soluble in water that is above the demarcation temperature, but insoluble in water that is below the demarcation temperature.
In one embodiment of the present invention, a method of making a raw batch for producing a composite ceramic body includes preparing a dry lubricant slurry and preparing a binder. The dry lubricant slurry and binder are mixed and dried to form a dry powder or mixture having a plurality of agglomerates. A majority of the agglomerates have a shape that is generally spherical.
In one aspect, the dry lubricant mixture is spray dried, although other drying methods also may be used within the scope of the present invention. In a particular aspect, the agglomerates have a size between about thirty (30) microns and about one hundred and twenty (120) microns. It will be appreciated by those skilled in the art that other sizes of agglomerates also fall within the scope of the present invention.
In one aspect, the method of making a raw batch mixture further includes mixing the dry mixture with a silicon carbide slurry. In one embodiment, the silicon carbide slurry includes a water based slurry. In one aspect, the water based slurry includes all needed sintering aids, disperants, lubricants, and binders to make a dense sintered silicon carbide matrix. In one aspect, the silicon carbide slurry includes alpha-silicon carbide. Alternatively, beta-silicon carbide may be used.
In one aspect, the silicon carbide slurry is less than about ninety (90) degrees Celsius. In another aspect, the silicon carbide slurry is less than about fifty (50) degrees Celsius. In this manner, the binder does not dissolve in the silicon carbide slurry, thus facilitating the retention of substantially spherical or ellipsoidal dry lubricant agglomerates.
In one aspect, the method further includes drying the combined dry mixture and silicon carbide slurry, and forming a ceramic body therefrom. This process may include, for example, green compaction forming techniques, machining with single point tooling, heating, firing or sintering the material, grinding with diamond tools, and the like. The ceramic body may comprise, for example, a pump seal, a bearing, a valve component, a turbine component, a pump lifter, a nozzle, or the like.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings.